We have isolated and partially purified inhibitors of adenylate cyclase and guanylate cyclase. The adenylate cyclase inhibitor, which is obtained from the in situ perfused rat liver acts on the catalytic site of the enzyme and non-competitively inhibits fluoride, hormone, cholera toxin, and guanyl nucleotide-stimulated adenylate cyclase activity from heart and other tissues. The inhibitor is acid-stable and heat labile, not species specific, and is also found in heart and kidney. It has currently been purified to near-homogeneity, and is being analyzed by gas chromatographic-mass spectroscopy. Once it is chemically and structurally identified we will produce an antibody against the inhibitor in order to develop a radioimmunoassay to determine tissue and serum concentrations of the substance. We will study the influence of a variety of physiologic and pathophysiologic states relative to cardiac function such as hormone (catecholamine, glucagon) stimulation, digitalization, and congestive heart failure. The guanylate cyclase inhibitor isolated from the balsam pear (Momoridica charantia abbreviata) has been purified about 30-fold, is thought to be a protein of approximate molecular weight 5,000 to 15,000, and is acid stable and heat labile. The inhibitor decreases guanylate cyclase activity from heart and other tissues, blocks the growth of rapidly growing cells and decreases the incorporation of (3H)-thymidine into DNA. The guanylate cyclase and adenylate cyclase inhibitors will be used as probes to determine which aspects of cardiac function are mediated by cyclic GMP and cyclic AMP respectively, which are not, and the precise relationship of the two nucleotides to cardiac function under physiologic and pathologic conditions. We have found that doxorubicin and daunorubicin selectively decrease cardiac guanylate cyclase activity, a finding which may help us to understand the molecular basis of the cardiotoxicity of these agents and the ultimate role of cyclic GMP in cardiac function. We will test a variety of analogues including some considered to be non-toxic to the heart.